In many situations it is desirable to be able to determine an absolute position on a surface. One example concerns the digitization of drawings. Another is when an electronic version of handwritten information is required.
U.S. Pat. No. 5,852,434 describes a device for determining an absolute position. The device comprises a writing surface which is provided with a position-coding pattern by means of which X-Y-coordinates can be determined, a detector which can detect the position-coding pattern and a processor which, on the basis of the detected position-coding pattern, can determine the position of the detector relative to the writing surface. The device makes it possible for a user to enter handwritten and hand-drawn information into a computer at the same time as the information is being written/drawn on the writing surface.
Three examples of position coding are given in U.S. Pat. No. 5,852,434. The first example is symbols, each of which is constructed of three concentric circles. The outer circle represents the X-coordinate and the middle circle the Y-coordinate. Both the outer circles are additionally divided into 16 parts which, depending upon whether they are filled in or not, indicate different numbers. This means that each pair of coordinates X, Y is coded by a complex symbol with a particular appearance.
In the second example the coordinates of each point on the writing surface are given by means of bar-codes, a bar-code for the X-coordinate being shown above a bar-code for the Y-coordinate.
A checkered pattern which can be used to code the X- and Y-coordinates is given as a third example. However, there is no explanation as to how the checkered pattern is constructed or how it can be converted into coordinates.
A problem with the known pattern is that it is constructed of complex symbols and the smaller these symbols are made, the more difficult it is to produce the patterned writing surface and the greater the risk of incorrect position determinations, while the larger the symbols are made, the poorer the position resolution becomes.
A further problem is that the processing of the detected position-coding pattern becomes rather complicated, due to the fact that a processor has to interpret complex symbols.
An additional problem is that the detector must be constructed in such a way that it can record four symbols at the same time so that it is certain to cover at least one symbol in its entirety, which is necessary in order for the position determination to be able to be carried out. The ratio between the required sensor surface and the surface of the position-coding pattern which defines a position is thus large.
Appendix A to WO 92/17859 gives the following example of how the pattern can be constructed and how a position can be decoded.
Take the following m-sequences: s=(0, 0, 1, 0, 1, 1, 1) and t=(0, 1, 1). Build up a position-coding pattern by letting a first column in the pattern be equal to the sequence s. In order to build up the following columns, look at the t-sequence. If the first element in the t-sequence is 0 then the second column consists of the s-sequence. If the first element is 1 instead, then the second column consists of the s-sequence cyclic-shifted by one step. Subsequent columns are built up in a corresponding way in accordance with the values of the elements in the t-sequence. The following pattern is then obtained:
0011000111000010110111101111
Assume now that one wants to find the position of a partial surface with the subset of the pattern shown below.
100010101
The first column in the subset is (1, 0, 1). This sub-sequence appears in position 2 in the s-sequence. The cyclic shifts in the subset are (1, 1). This sub-sequence appears in position 1 in the t-sequence. The accumulated shifts in the pattern is (0, 0, 1, 2) and therefore the vertical position of the subset is 2+0=2. The position of the subset on the partial surface is thus (1, 2).
With this pattern the above-mentioned problems with complex symbols are avoided and the ratio is reduced between the required sensor surface and the surface of the position-coding pattern which defines a position.
An interesting characteristic of a position-coding pattern of this type is, however, the ability to code a large pattern with many unique positions so that position determination can be carried out on as large a surface as possible. In the example described above, the size in the vertical direction is limited by the length of the s-sequence and the size in the horizontal direction by the length of the t-sequence. The length of these sequences can, however, not be increased without limit as the sequences should have the characteristic that if a sub-sequence of k bits is taken, this sub-sequence should only occur in one position in the sequence. An increase in the length of the sequence can thus imply an increase in the length of the sub-sequence and thereby an increase of the partial surface which must be recorded in order to be able to determine a position.